Oracle Interview Questions And Answers 4dx37
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Oracle Interview Questions and Answers : SQL 1. To see current name Sql> show ; 2. Change SQL prompt name SQL> set sqlprompt “Manimara > “ 3. Switch to DOS prompt SQL> host 4. How do I eliminate the duplicate rows ? SQL> delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name); or SQL> delete duplicate_values_field_name dv from table_name ta where rowid <(select min(rowid) from table_name tb where ta.dv=tb.dv); Example. Table Emp Empno Ename 101 Scott 102 Jiyo 103 Millor 104 Jiyo 105 Smith delete ename from emp a where rowid < ( select min(rowid) from emp b where a.ename = b.ename); The output like, Empno Ename 101 Scott 102 Millor 103 Jiyo 104 Smith 5. How do I display row number with records? To achive this use rownum pseudocolumn with query, like SQL> SQL> select rownum, ename from emp; Output: 1 Scott 2 Millor 3 Jiyo 4 Smith 6. Display the records between two range select rownum, empno, ename from emp where rowid in (select rowid from emp where rownum <=&upto minus select rowid from emp where rownum<&Start); Enter value for upto: 10 Enter value for Start: 7 ROWNUM EMPNO ENAME --------- --------- ----------
1 2 3 4
7782 CLARK 7788 SCOTT 7839 KING 7844 TURNER
7. I know the nvl function only allows the same data type (i.e. number or char or date Nvl(comm, 0)), if commission is null then the text “Not Applicable” want to display, instead of blank space. How do I write the query? SQL> select nvl(to_char(comm.),'NA') from emp; Output : NVL(TO_CHAR(COMM),'NA') ----------------------NA 300 500 NA 1400 NA NA 8. Oracle cursor : Implicit & Explicit cursors Oracle uses work areas called private SQL areas to create SQL statements. PL/SQL construct to identify each and every work are used, is called as Cursor. For SQL queries returning a single row, PL/SQL declares all implicit cursors. For queries that returning more than one row, the cursor needs to be explicitly declared. 9. Explicit Cursor attributes There are four cursor attributes used in Oracle cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN 10. Implicit Cursor attributes Same as explicit cursor but prefixed by the word SQL SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN Tips : 1. Here SQL%ISOPEN is false, because oracle automatically closed the implicit cursor after executing SQL statements. : 2. All are Boolean attributes. 11. Find out nth highest salary from emp table SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal); Enter value for n: 2 SAL --------3700
12. To view installed Oracle version information SQL> select banner from v$version; 13. Display the number value in Words SQL> select sal, (to_char(to_date(sal,'j'), 'jsp')) from emp; the output like, SAL (TO_CHAR(TO_DATE(SAL,'J'),'JSP')) --------- ----------------------------------------------------800 eight hundred 1600 one thousand six hundred 1250 one thousand two hundred fifty If you want to add some text like, Rs. Three Thousand only. SQL> select sal "Salary ", (' Rs. '|| (to_char(to_date(sal,'j'), 'Jsp'))|| ' only.')) "Sal in Words" from emp / Salary Sal in Words ------- -----------------------------------------------------800 Rs. Eight Hundred only. 1600 Rs. One Thousand Six Hundred only. 1250 Rs. One Thousand Two Hundred Fifty only. 14. Display Odd/ Even number of records Odd number of records: select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp); 1 3 5 Even number of records: select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp); 2 4 6 15. Which date function returns number value? months_between 16. Any three PL/SQL Exceptions? Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others 17. What are PL/SQL Cursor Exceptions? Cursor_Already_Open, Invalid_Cursor 18. Other way to replace query result null value with a text SQL> Set NULL ‘N/A’ to reset SQL> Set NULL ‘’ 19. What are the more common pseudo-columns? SYSDATE, , UID, CURVAL, NEXTVAL, ROWID, ROWNUM
20. What is the output of SIGN function? 1 for positive value, 0 for Zero, -1 for Negative value. 21. What is the maximum number of triggers, can apply to a single table? 12 triggers. 4.
Explain the difference between a data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object. 5.
Give two examples of how you might determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package. 6.
Where would you look for errors from the database engine?
In the alert log. 7.
Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a no rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete. 8.
Give the reasoning behind using an index.
Faster access to data blocks in a table. 9. Give the two types of tables involved in producing a star schema and the type of data they hold. Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables. 10. . What type of index should you use on a fact table? A Bitmap index. 11. Give two examples of referential integrity constraints. A primary key and a foreign key. 12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables? Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.
13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each. ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly. 14. What command would you use to create a backup control file? Alter database backup control file to trace. 15. Give the stages of instance startup to a usable state where normal s may access it. STARTUP NOMOUNT - Instance startup STARTUP MOUNT - The database is mounted STARTUP OPEN - The database is opened 16. What column differentiates the V$ views to the GV$ views and how? The INST_ID column which indicates the instance in a RAC environment the information came from. 17. How would you go about generating an EXPLAIN plan? Create a plan table with utlxplan.sql. Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement Look at the explain plan with utlxplp.sql or utlxpls.sql 18. How would you go about increasing the buffer cache hit ratio? Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command. 19. Explain an ORA-01555 You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message. 20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE. ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside. 21. How would you determine the time zone under which a database was operating? select DBTIMEZONE from dual; 22. Explain the use of setting GLOBAL_NAMES equal to TRUE.
Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking. 23. What command would you use to encrypt a PL/SQL application? WRAP 24. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE. A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application. 25. Explain the use of table functions. Table functions are designed to return a set of rows through PL/SQL logic but are intended to be used as a normal table or view in a SQL statement. They are also used to pipeline information in an ETL process. 26. Name three advisory statistics you can collect. Buffer Cache Advice, Segment Level Statistics, & Timed Statistics 27. Where in the Oracle directory tree structure are audit traces placed? In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer 28. Explain materialized views and how they are used. Materialized views are objects that are reduced sets of information that have been summarized, grouped, or aggregated from base tables. They are typically used in data warehouse or decision systems. 29. When a process fails, what background process cleans up after it? PMON 30. What background process refreshes materialized views? The Job Queue Processes. 31. How would you determine what sessions are connected and what resources they are waiting for? Use of V$SESSION and V$SESSION_WAIT 32. Describe what redo logs are. Redo logs are logical and physical structures that are designed to hold all the changes made to a database and are intended to aid in the recovery of a database. 33. How would you force a log switch?
ALTER SYSTEM SWITCH LOGFILE; 34. Give two methods you could use to determine what DDL changes have been made. You could use Logminer or Streams 35. What does coalescing a tablespace do? Coalescing is only valid for dictionary-managed tablespaces and de-fragments space by combining neighboring free extents into large single extents. 36. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace? A temporary tablespace is used for temporary objects such as sort structures while permanent tablespaces are used to store those objects meant to be used as the true objects of the database. 37. Name a tablespace automatically created when you create a database. The SYSTEM tablespace. 38. When creating a , what permissions must you grant to allow them to connect to the database? Grant the CONNECT to the . 39. How do you add a data file to a tablespace? ALTER TABLESPACE
ADD DATAFILE
SIZE <size> 40. How do you resize a data file? ALTER DATABASE DATAFILE
RESIZE
; 41. What view would you use to look at the size of a data file? DBA_DATA_FILES 42. What view would you use to determine free space in a tablespace? DBA_FREE_SPACE 43. How would you determine who has added a row to a table? Turn on fine grain auditing for the table. 44. How can you rebuild an index? ALTER INDEX
REBUILD; 45. Explain what partitioning is and what its benefit is. Partitioning is a method of taking large tables and indexes and splitting them into smaller, more manageable pieces.
46. You have just compiled a PL/SQL package but got errors, how would you view the errors? SHOW ERRORS 47. How can you gather statistics on a table? The ANALYZE command. 48. How can you enable a trace for a session? Use the DBMS_SESSION.SET_SQL_TRACE or Use ALTER SESSION SET SQL_TRACE = TRUE; 49. What is the difference between the SQL*Loader and IMPORT utilities? These two Oracle utilities are used for loading data into the database. The difference is that the import utility relies on the data being produced by another Oracle utility EXPORT while the SQL*Loader utility allows data to be loaded that has been produced by other utilities from different data sources just so long as it conforms to ASCII formatted or delimited files. 50. Name two files used for network connection to a database. TNSNAMES.ORA and SQLNET.ORA
Oracle Concepts and Architecture Database Structures 1. What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files. 2. What are the components of logical database structure of Oracle database?
There are tablespaces and database's schema objects. 3. What is a tablespace?
A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together. 4. What is SYSTEM tablespace and when is it created?
Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database. 5. Explain the relationship among database, tablespace and data file.
Each databases logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace. 6. What is schema?
A schema is collection of database objects of a . 7. What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links. 8. Can objects of the same schema reside in different tablespaces?
Yes. 9. Can a tablespace hold objects from different schemes?
Yes. 10. What is Oracle table?
A table is the basic unit of data storage in an Oracle database. The tables of a database hold all of the accessible data. Table data is stored in rows and columns. 11. What is an Oracle view?
A view is a virtual table. Every view has a query attached to it. (The query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.) 12. Do a view contain data? Views do not contain or store data. 13. Can a view based on another view?
Yes. 14. What are the advantages of views? - Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table. - Hide data complexity. - Simplify commands for the . - Present the data in a different perspective from that of the base table.
- Store complex queries. 15. What is an Oracle sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database's tables. 16. What is a synonym?
A synonym is an alias for a table, view, sequence or program unit. 17. What are the types of synonyms? There are two types of synonyms private and public.
18. What is a private synonym? Only its owner can access a private synonym. 19. What is a public synonym? Any database can access a public synonym. 20. What are synonyms used for? - Mask the real name and owner of an object.
- Provide public access to an object - Provide location transparency for tables, views or program units of a remote database. - Simplify the SQL statements for database s. 21. What is an Oracle index?
An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table. 22. How are the index updates?
Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes. 23. What are clusters?
Clusters are groups of one or more tables physically stores together to share common columns and are often used together. 24. What is cluster key? The related columns of the tables in a cluster are called the cluster key. 25. What is index cluster?
A cluster with an index on the cluster key. 26. What is hash cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk. 27. When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows. 28. What is database link?
A database link is a named object that describes a "path" from one database to another.
29. What are the types of database links?
Private database link, public database link & network database link. 30. What is private database link?
Private database link is created on behalf of a specific . A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures. 31. What is public database link?
Public database link is created for the special group PUBLIC. A public database link can be used when any in the associated database specifies a global object name in a SQL statement or object definition. 32. What is network database link?
Network database link is created and managed by a network domain service. A network database link can be used when any of any database in the network specifies a global object name in a SQL statement or object definition. 33. What is data block?
Oracle database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. 34. How to define data block size?
A data block size is specified for each Oracle database when the database is created. A database s and allocated free database space in Oracle data blocks. Block size is specified in init.ora file and cannot be changed latter. 35. What is row chaining?
In circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment. 36. What is an extent?
An extent is a specific number of contiguous data blocks, obtained in a single allocation and used to store a specific type of information. 37. What is a segment?
A segment is a set of extents allocated for a certain logical structure. 38. What are the different types of segments?
Data segment, index segment, rollback segment and temporary segment. 39. What is a data segment?
Each non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment. 40. What is an index segment?
Each index has an index segment that stores all of its data. 41. What is rollback segment?
A database contains one or more rollback segments to temporarily store "undo" information. 42. What are the uses of rollback segment? To generate read-consistent database information during database recovery and to rollback uncommitted transactions by the s. 43. What is a temporary segment?
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use. 44. What is a datafile?
Every Oracle database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database. 45. What are the characteristics of data files? A data file can be associated with only one database. Once created a data file can't change size. One or more data files form a logical unit of database storage called a tablespace. 46. What is a redo log?
The set of redo log files for a database is collectively known as the database redo log. 47. What is the function of redo log?
The primary function of the redo log is to record all changes made to data. 48. What is the use of redo formation?
The information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files. 49. What does a control file contains? - Database name - Names and locations of a database's files and redolog files. - Time stamp of database creation. 50. What is the use of control file?
When an instance of an Oracle database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery. Data Base istration 51. What is a database instance? Explain.
A database instance (Server) is a set of memory structure and background processes that access a set of database files. The processes can be shared by all of the s. The memory structure that is used to store the most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file. 52. What is Parallel Server?
Multiple instances accessing the same database (only in multi-U environments) 53. What is a schema?
The set of objects owned by is called the schema. 54. What is an index? How it is implemented in Oracle database? An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table command 55. What are clusters?
Group of tables physically stored together because they share common columns and are often used together is called cluster. 56. What is a cluster key?
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster. 57. What are the basic element of base configuration of an Oracle database? It consists of one or more data files.
one or more control files. two or more redo log files. The Database contains multiple s/schemas one or more rollback segments one or more tablespaces Data dictionary tables objects (table,indexes,views etc.,) The server that access the database consists of SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool) SMON (System MONito) PMON (Process MONitor) LGWR (LoG Write) DBWR (Data Base Write)
ARCH (ARCHiver) CKPT (Check Point) RECO Dispatcher Process with associated PGS 58. What is a deadlock? Explain. Two processes waiting to update the rows of a table, which are locked by other processes then deadlock arises. In a database environment this will often happen because of not issuing the proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically. These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally. Memory Management 59. What is SGA? The System Global Area in an Oracle database is the area in memory to facilitate the transfer of information between s. It holds the most recently requested structural information between s. It holds the most recently requested structural information about the database. The structure is database buffers, dictionary cache, redo log buffer and shared pool area. 60. What is a shared pool?
The data dictionary cache is stored in an area in SGA called the shared pool. This will allow sharing of parsed SQL statements among concurrent s. 61. What is mean by Program Global Area (PGA)?
It is area in memory that is used by a single Oracle process. 62. What is a data segment?
Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored. 63. What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient shared pool size. Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE. Database Logical & Physical Architecture 64. What is Database Buffers?
Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
65. What is dictionary cache?
Dictionary cache is information about the database objects stored in a data dictionary table. 66. What is meant by recursive hints?
Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of data dictionary cache. 67. What is redo log buffer?
Changes made to the records are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size. 68. How will you swap objects into a different table space for an existing database? - Export the - Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql. - Drop necessary objects. - Run the script newfile.sql after altering the tablespaces. - Import from the backup for the necessary objects. 69. List the Optional Flexible Architecture (OFA) of Oracle database? How can we organize the tablespaces in Oracle database to have maximum performance? SYSTEM - Data dictionary tables. DATA - Standard operational tables. DATA2- Static tables used for standard operations INDEXES - Indexes for Standard operational tables. INDEXES1 - Indexes of static tables used for standard operations. TOOLS - Tools table. TOOLS1 - Indexes for tools table. RBS - Standard Operations Rollback Segments, RBS1,RBS2 - Additional/Special Rollback segments. TEMP - Temporary purpose tablespace TEMP_ - Temporary tablespace for s. S - tablespace. 70. How will you force database to use particular rollback segment?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name. 71. What is meant by free extent? A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free. 72.Which parameter in Storage clause will reduce number of rows per block? PCTFREE parameter
Row size also reduces no of rows per block. 73. What is the significance of having storage clause?
We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updating, etc., 74. How does Space allocation table place within a block? Each block contains entries as follows Fixed block header Variable block header Row Header, row date (multiple rows may exists)
PCTEREE (% of free space for row updating in future) 75. What is the role of PCTFREE parameter is storage clause?
This is used to reserve certain amount of space in a block for expansion of rows. 76. What is the OPTIMAL parameter?
It is used to set the optimal length of a rollback segment. 77. What is the functionality of SYSTEM table space?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage. 78. How will you create multiple rollback segments in a database?
- Create a database, which implicitly creates a SYSTEM rollback segment in a SYSTEM tablespace. - Create a second rollback segment name R0 in the SYSTEM tablespace. - Make new rollback segment available (after shutdown, modify init.ora file and start database) - Create other tablespaces (RBS) for rollback segments. - Deactivate rollback segment R0 and activate the newly created rollback segments. 79. How the space utilization takes place within rollback segments?
It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (number of extents is based on the optimal size) 80. Why query fails sometimes? Rollback segment dynamically extent to handle larger transactions entry loads. A single transaction may wipeout all available free space in the rollback segment tablespace. This prevents other using rollback segments. 81. How will you monitor the space allocation?
By querying DBA_SEGMENT table/view
82. How will you monitor rollback segment status? Querying the DBA_ROLLBACK_SEGS view IN USE - Rollback Segment is on-line. AVAILABLE - Rollback Segment available but not on-line. OFF-LINE - Rollback Segment off-line INVALID - Rollback Segment Dropped. NEEDS RECOVERY - Contains data but need recovery or corrupted. PARTLY AVAILABLE - Contains data from an unresolved transaction involving a distributed database. 83. List the sequence of events when a large transaction that exceeds beyond its optimal value when an entry wraps and causes the rollback segment to expand into another extend. Transaction Begins. An entry is made in the RES header for new transactions entry Transaction acquires blocks in an extent of RBS The entry attempts to wrap into second extent. None is available, so that the RBS must extent. The RBS checks to see if it is part of its OPTIMAL size. RBS chooses its oldest inactive segment. Oldest inactive segment is eliminated. RBS extents The data dictionary tables for space management are updated. Transaction Completes. 84. How can we plan storage for very large tables? Limit the number of extents in the table
Separate table from its indexes. Allocate sufficient temporary storage. 85. How will you estimate the space required by a non-clustered tables? Calculate the total header size Calculate the available data space per data block Calculate the combined column lengths of the average row Calculate the total average row size. Calculate the average number rows that can fit in a block Calculate the number of blocks and bytes required for the table. After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table. 86. It is possible to use raw devices as data files and what are the advantages over file system files? Yes. The advantages over file system files are that I/O will be improved because Oracle is bye-ing the kernel which writing into disk. Disk corruption will be very less.
87. What is a Control file? Database's overall physical architecture is maintained in a file called control file. It will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable. 88. How to implement the multiple control files for an existing database? Shutdown the database Copy one of the existing controlfile to new location Edit Config ora file by adding new control filename Restart the database. 89. What is redo log file mirroring? How can be achieved? Process of having a copy of redo log files is called mirroring. This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance. 90. What is advantage of having disk shadowing / mirroring? Shadow set of disks save as a backup in the event of disk failure. In most operating systems if any disk failure occurs it automatically switchover to place of failed disk. Improved performance because most OS volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks. 91. What is use of rollback segments in Oracle database?
They allow the database to maintain read consistency between multiple transactions. 92. What is a rollback segment entry? It is the set of before image data blocks that contain rows that are modified by a transaction.
Each rollback segment entry must be completed within one rollback segment. A single rollback segment can have multiple rollback segment entries. 93. What is hit ratio? It is a measure of well the data cache buffer is handling requests for data. Hit Ratio = (Logical Reads - Physical Reads - Hits Misses)/ Logical Reads. 94. When will be a segment released?
When Segment is dropped. When Shrink (RBS only) When truncated (TRUNCATE used with drop storage option) 95. What are disadvantages of having raw devices? We should depend on export/import utility for backup/recovery (fully reliable)
The tar command cannot be used for physical file backup, instead we can use dd command, which is less flexible and has limited recoveries. 96. List the factors that can affect the accuracy of the estimations? - The space used transaction entries and deleted records, does not become free immediately after completion due to delayed cleanout. - Trailing nulls and length bytes are not stored. - Inserts of, updates to and deletes of rows as well as columns larger than a single data block, can cause fragmentation a chained row pieces.
SQL*Plus Statements 100. What are the types of SQL statement? Data Definition Language: CREATE, ALTER, DROP, TRUNCATE, REVOKE, NO AUDIT & COMMIT. Data Manipulation Language: INSERT, UPDATE, DELETE, LOCK TABLE, EXPLAIN PLAN & SELECT. Transactional Control: COMMIT & ROLLBACK Session Control: ALTERSESSION & SET ROLE System Control: ALTER SYSTEM. 101. What is a transaction?
Transaction is logical unit between two commits and commit and rollback. 102. What is difference between TRUNCATE & DELETE? TRUNCATE commits after deleting entire table i.e., cannot be rolled back. Database triggers do not fire on TRUNCATE
DELETE allows the filtered deletion. Deleted records can be rolled back or committed. Database triggers fire on DELETE. 103. What is a ? Explain the different types of s? is a query, which retrieves related columns or rows from multiple tables. Self - ing the table with itself. Equi - ing two tables by equating two common columns. Non-Equi - ing two tables by equating two common columns. Outer - ing two tables in such a way that query can also retrieve rows that do not have corresponding value in the other table. 104. What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query. 105. What is correlated sub-query? Correlated sub-query is a sub-query, which has reference to the main query. 106. Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg. select empno, ename from emp where. 107. Difference between SUBSTR and INSTR? INSTR (String1, String2 (n, (m)), INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1. SUBSTR (String1 n, m) SUBSTR returns a character string of size m in string1, starting from n-th position of string1. 108. Explain UNION, MINUS, UNION ALL and INTERSECT? INTERSECT MINUS UNION UNION ALL
- returns all distinct rows selected by both queries. - returns all distinct rows selected by the first query but not by the second. - returns all distinct rows selected by either query - returns all rows selected by either query, including all duplicates.
109. What is ROWID?
ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID. 110. What is the fastest way of accessing a row in a table? Using ROWID. CONSTRAINTS 111. What is an integrity constraint?
Integrity constraint is a rule that restricts values to a column in a table. 112. What is referential integrity constraint?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table. 113. What is the usage of SAVEPOINTS?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed. 114. What is ON DELETE CASCADE?
When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed. 115. What are the data types allowed in a table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW. 116. What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2. 117. How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?
Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause. 118. What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint? - To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty. 119. Where the integrity constraints are stored in data dictionary?
The integrity constraints are stored in _CONSTRAINTS. 120. How will you activate/deactivate integrity constraints?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT. 121. If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE?
It won't, Because SYSDATE format contains time attached with it. 122. What is a database link?
Database link is a named path through which a remote database can be accessed. 123. How to access the current value and next value from a sequence? Is it possible to access the current value in a session before accessing next value?
Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed. 124. What is CYCLE/NO CYCLE in a Sequence?
CYCLE specifies that the sequence continue to generate values after reaching either maximum or minimum value. After pan-ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum. NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value. 125. What are the advantages of VIEW? - To protect some of the columns of a table from other s. - To hide complexity of a query. - To hide complexity of calculations.
126. Can a view be updated/inserted/deleted? If Yes - under what conditions?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible. 127. If a view on a single base table is manipulated will the changes be reflected on the base table?
If changes are made to the tables and these tables are the base tables of a view, then the changes will be reference on the view. Q) What are the Various Master and Detail Relation ships. A) The various Master and a) NonIsolated :: b) Isolated :: c) Cascading ::
Detail Relationship are The Master cannot be deleted when a child is exisiting The Master can be deleted when the child is exisiting The child gets deleted when the Master is deleted.
Q) What is the Difference between a post query and a pre query A) A post query will fire for every row that is fetched but the pre query will fire only once. Q) How do you implement one-to-one, one-to-many and many-to-many relationships while deg tables? A) One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. Q) Can the following Query be Optimized? SELECT CID FROM Course WHERE title LIKE ’S%’ AND min_enroll > (SELECT COUNT(*) FROM Enroll WHERE Enroll.CID = Course.CID); A) The query is very expensive,s ince it is evaluated once for every S course, yes it can be optimised via the followig • Process the outer query without the subquery - CREATE VIEW Supp_Course AS SElect * from Course Where title like 'S%' • Collect bindings – CREATE VIEW Magic AS SELECT DISTINCT CID FROM Supp_Course; • Evaluate the subquery with bindings – CREATE VIEW DS AS sELECT Enroll.CID, COUNT(*) AS cnt FROM Magic, Enroll WHERE Magic.CID = Enroll.CID GROUP BY Enroll.CID; UNION SELECT Magic.CID, 0 AS cnt -- the COUNT patch FROM Magic WHERE Magic.CID NOT IN(SELECT CID FROM Enroll);
• Finally, refine the outer query – SELECT Supp_Course.CID FROM Supp_Course, DS WHERE Supp_Course.CID = DS.CID AND min_enroll > DS.cnt; Q) Which one of the two queries gives the best performance (using sub-q) select person,birth_yr from data where birth_yr in (select birth_yr from data group by birth_yr having count(person) >1 ) (using ) select a.person, b.person, a.birth_yr from data a, data b where a.birth_yr = b.birth_yr and a.person < b.person A) in the above scenario, the subquery gives the best performance since the iterates for each person affecting performance.
1 2 3 4
7782 CLARK 7788 SCOTT 7839 KING 7844 TURNER
7. I know the nvl function only allows the same data type (i.e. number or char or date Nvl(comm, 0)), if commission is null then the text “Not Applicable” want to display, instead of blank space. How do I write the query? SQL> select nvl(to_char(comm.),'NA') from emp; Output : NVL(TO_CHAR(COMM),'NA') ----------------------NA 300 500 NA 1400 NA NA 8. Oracle cursor : Implicit & Explicit cursors Oracle uses work areas called private SQL areas to create SQL statements. PL/SQL construct to identify each and every work are used, is called as Cursor. For SQL queries returning a single row, PL/SQL declares all implicit cursors. For queries that returning more than one row, the cursor needs to be explicitly declared. 9. Explicit Cursor attributes There are four cursor attributes used in Oracle cursor_name%Found, cursor_name%NOTFOUND, cursor_name%ROWCOUNT, cursor_name%ISOPEN 10. Implicit Cursor attributes Same as explicit cursor but prefixed by the word SQL SQL%Found, SQL%NOTFOUND, SQL%ROWCOUNT, SQL%ISOPEN Tips : 1. Here SQL%ISOPEN is false, because oracle automatically closed the implicit cursor after executing SQL statements. : 2. All are Boolean attributes. 11. Find out nth highest salary from emp table SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT (DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal); Enter value for n: 2 SAL --------3700
12. To view installed Oracle version information SQL> select banner from v$version; 13. Display the number value in Words SQL> select sal, (to_char(to_date(sal,'j'), 'jsp')) from emp; the output like, SAL (TO_CHAR(TO_DATE(SAL,'J'),'JSP')) --------- ----------------------------------------------------800 eight hundred 1600 one thousand six hundred 1250 one thousand two hundred fifty If you want to add some text like, Rs. Three Thousand only. SQL> select sal "Salary ", (' Rs. '|| (to_char(to_date(sal,'j'), 'Jsp'))|| ' only.')) "Sal in Words" from emp / Salary Sal in Words ------- -----------------------------------------------------800 Rs. Eight Hundred only. 1600 Rs. One Thousand Six Hundred only. 1250 Rs. One Thousand Two Hundred Fifty only. 14. Display Odd/ Even number of records Odd number of records: select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp); 1 3 5 Even number of records: select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp); 2 4 6 15. Which date function returns number value? months_between 16. Any three PL/SQL Exceptions? Too_many_rows, No_Data_Found, Value_Error, Zero_Error, Others 17. What are PL/SQL Cursor Exceptions? Cursor_Already_Open, Invalid_Cursor 18. Other way to replace query result null value with a text SQL> Set NULL ‘N/A’ to reset SQL> Set NULL ‘’ 19. What are the more common pseudo-columns? SYSDATE, , UID, CURVAL, NEXTVAL, ROWID, ROWNUM
20. What is the output of SIGN function? 1 for positive value, 0 for Zero, -1 for Negative value. 21. What is the maximum number of triggers, can apply to a single table? 12 triggers. 4.
Explain the difference between a data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object. 5.
Give two examples of how you might determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package. 6.
Where would you look for errors from the database engine?
In the alert log. 7.
Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a no rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete. 8.
Give the reasoning behind using an index.
Faster access to data blocks in a table. 9. Give the two types of tables involved in producing a star schema and the type of data they hold. Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables. 10. . What type of index should you use on a fact table? A Bitmap index. 11. Give two examples of referential integrity constraints. A primary key and a foreign key. 12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables? Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.
13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each. ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly. 14. What command would you use to create a backup control file? Alter database backup control file to trace. 15. Give the stages of instance startup to a usable state where normal s may access it. STARTUP NOMOUNT - Instance startup STARTUP MOUNT - The database is mounted STARTUP OPEN - The database is opened 16. What column differentiates the V$ views to the GV$ views and how? The INST_ID column which indicates the instance in a RAC environment the information came from. 17. How would you go about generating an EXPLAIN plan? Create a plan table with utlxplan.sql. Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement Look at the explain plan with utlxplp.sql or utlxpls.sql 18. How would you go about increasing the buffer cache hit ratio? Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command. 19. Explain an ORA-01555 You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message. 20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE. ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside. 21. How would you determine the time zone under which a database was operating? select DBTIMEZONE from dual; 22. Explain the use of setting GLOBAL_NAMES equal to TRUE.
Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking. 23. What command would you use to encrypt a PL/SQL application? WRAP 24. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE. A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application. 25. Explain the use of table functions. Table functions are designed to return a set of rows through PL/SQL logic but are intended to be used as a normal table or view in a SQL statement. They are also used to pipeline information in an ETL process. 26. Name three advisory statistics you can collect. Buffer Cache Advice, Segment Level Statistics, & Timed Statistics 27. Where in the Oracle directory tree structure are audit traces placed? In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer 28. Explain materialized views and how they are used. Materialized views are objects that are reduced sets of information that have been summarized, grouped, or aggregated from base tables. They are typically used in data warehouse or decision systems. 29. When a process fails, what background process cleans up after it? PMON 30. What background process refreshes materialized views? The Job Queue Processes. 31. How would you determine what sessions are connected and what resources they are waiting for? Use of V$SESSION and V$SESSION_WAIT 32. Describe what redo logs are. Redo logs are logical and physical structures that are designed to hold all the changes made to a database and are intended to aid in the recovery of a database. 33. How would you force a log switch?
ALTER SYSTEM SWITCH LOGFILE; 34. Give two methods you could use to determine what DDL changes have been made. You could use Logminer or Streams 35. What does coalescing a tablespace do? Coalescing is only valid for dictionary-managed tablespaces and de-fragments space by combining neighboring free extents into large single extents. 36. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace? A temporary tablespace is used for temporary objects such as sort structures while permanent tablespaces are used to store those objects meant to be used as the true objects of the database. 37. Name a tablespace automatically created when you create a database. The SYSTEM tablespace. 38. When creating a , what permissions must you grant to allow them to connect to the database? Grant the CONNECT to the . 39. How do you add a data file to a tablespace? ALTER TABLESPACE
46. You have just compiled a PL/SQL package but got errors, how would you view the errors? SHOW ERRORS 47. How can you gather statistics on a table? The ANALYZE command. 48. How can you enable a trace for a session? Use the DBMS_SESSION.SET_SQL_TRACE or Use ALTER SESSION SET SQL_TRACE = TRUE; 49. What is the difference between the SQL*Loader and IMPORT utilities? These two Oracle utilities are used for loading data into the database. The difference is that the import utility relies on the data being produced by another Oracle utility EXPORT while the SQL*Loader utility allows data to be loaded that has been produced by other utilities from different data sources just so long as it conforms to ASCII formatted or delimited files. 50. Name two files used for network connection to a database. TNSNAMES.ORA and SQLNET.ORA
Oracle Concepts and Architecture Database Structures 1. What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files. 2. What are the components of logical database structure of Oracle database?
There are tablespaces and database's schema objects. 3. What is a tablespace?
A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together. 4. What is SYSTEM tablespace and when is it created?
Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database. 5. Explain the relationship among database, tablespace and data file.
Each databases logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace. 6. What is schema?
A schema is collection of database objects of a . 7. What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links. 8. Can objects of the same schema reside in different tablespaces?
Yes. 9. Can a tablespace hold objects from different schemes?
Yes. 10. What is Oracle table?
A table is the basic unit of data storage in an Oracle database. The tables of a database hold all of the accessible data. Table data is stored in rows and columns. 11. What is an Oracle view?
A view is a virtual table. Every view has a query attached to it. (The query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.) 12. Do a view contain data? Views do not contain or store data. 13. Can a view based on another view?
Yes. 14. What are the advantages of views? - Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table. - Hide data complexity. - Simplify commands for the . - Present the data in a different perspective from that of the base table.
- Store complex queries. 15. What is an Oracle sequence?
A sequence generates a serial list of unique numbers for numerical columns of a database's tables. 16. What is a synonym?
A synonym is an alias for a table, view, sequence or program unit. 17. What are the types of synonyms? There are two types of synonyms private and public.
18. What is a private synonym? Only its owner can access a private synonym. 19. What is a public synonym? Any database can access a public synonym. 20. What are synonyms used for? - Mask the real name and owner of an object.
- Provide public access to an object - Provide location transparency for tables, views or program units of a remote database. - Simplify the SQL statements for database s. 21. What is an Oracle index?
An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table. 22. How are the index updates?
Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes. 23. What are clusters?
Clusters are groups of one or more tables physically stores together to share common columns and are often used together. 24. What is cluster key? The related columns of the tables in a cluster are called the cluster key. 25. What is index cluster?
A cluster with an index on the cluster key. 26. What is hash cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the row's cluster key value. All rows with the same hash key value are stores together on disk. 27. When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows. 28. What is database link?
A database link is a named object that describes a "path" from one database to another.
29. What are the types of database links?
Private database link, public database link & network database link. 30. What is private database link?
Private database link is created on behalf of a specific . A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures. 31. What is public database link?
Public database link is created for the special group PUBLIC. A public database link can be used when any in the associated database specifies a global object name in a SQL statement or object definition. 32. What is network database link?
Network database link is created and managed by a network domain service. A network database link can be used when any of any database in the network specifies a global object name in a SQL statement or object definition. 33. What is data block?
Oracle database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk. 34. How to define data block size?
A data block size is specified for each Oracle database when the database is created. A database s and allocated free database space in Oracle data blocks. Block size is specified in init.ora file and cannot be changed latter. 35. What is row chaining?
In circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment. 36. What is an extent?
An extent is a specific number of contiguous data blocks, obtained in a single allocation and used to store a specific type of information. 37. What is a segment?
A segment is a set of extents allocated for a certain logical structure. 38. What are the different types of segments?
Data segment, index segment, rollback segment and temporary segment. 39. What is a data segment?
Each non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment. 40. What is an index segment?
Each index has an index segment that stores all of its data. 41. What is rollback segment?
A database contains one or more rollback segments to temporarily store "undo" information. 42. What are the uses of rollback segment? To generate read-consistent database information during database recovery and to rollback uncommitted transactions by the s. 43. What is a temporary segment?
Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use. 44. What is a datafile?
Every Oracle database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database. 45. What are the characteristics of data files? A data file can be associated with only one database. Once created a data file can't change size. One or more data files form a logical unit of database storage called a tablespace. 46. What is a redo log?
The set of redo log files for a database is collectively known as the database redo log. 47. What is the function of redo log?
The primary function of the redo log is to record all changes made to data. 48. What is the use of redo formation?
The information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files. 49. What does a control file contains? - Database name - Names and locations of a database's files and redolog files. - Time stamp of database creation. 50. What is the use of control file?
When an instance of an Oracle database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery. Data Base istration 51. What is a database instance? Explain.
A database instance (Server) is a set of memory structure and background processes that access a set of database files. The processes can be shared by all of the s. The memory structure that is used to store the most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file. 52. What is Parallel Server?
Multiple instances accessing the same database (only in multi-U environments) 53. What is a schema?
The set of objects owned by is called the schema. 54. What is an index? How it is implemented in Oracle database? An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table command 55. What are clusters?
Group of tables physically stored together because they share common columns and are often used together is called cluster. 56. What is a cluster key?
The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster. 57. What are the basic element of base configuration of an Oracle database? It consists of one or more data files.
one or more control files. two or more redo log files. The Database contains multiple s/schemas one or more rollback segments one or more tablespaces Data dictionary tables objects (table,indexes,views etc.,) The server that access the database consists of SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool) SMON (System MONito) PMON (Process MONitor) LGWR (LoG Write) DBWR (Data Base Write)
ARCH (ARCHiver) CKPT (Check Point) RECO Dispatcher Process with associated PGS 58. What is a deadlock? Explain. Two processes waiting to update the rows of a table, which are locked by other processes then deadlock arises. In a database environment this will often happen because of not issuing the proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically. These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally. Memory Management 59. What is SGA? The System Global Area in an Oracle database is the area in memory to facilitate the transfer of information between s. It holds the most recently requested structural information between s. It holds the most recently requested structural information about the database. The structure is database buffers, dictionary cache, redo log buffer and shared pool area. 60. What is a shared pool?
The data dictionary cache is stored in an area in SGA called the shared pool. This will allow sharing of parsed SQL statements among concurrent s. 61. What is mean by Program Global Area (PGA)?
It is area in memory that is used by a single Oracle process. 62. What is a data segment?
Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored. 63. What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient shared pool size. Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE. Database Logical & Physical Architecture 64. What is Database Buffers?
Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
65. What is dictionary cache?
Dictionary cache is information about the database objects stored in a data dictionary table. 66. What is meant by recursive hints?
Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of data dictionary cache. 67. What is redo log buffer?
Changes made to the records are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size. 68. How will you swap objects into a different table space for an existing database? - Export the - Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql. - Drop necessary objects. - Run the script newfile.sql after altering the tablespaces. - Import from the backup for the necessary objects. 69. List the Optional Flexible Architecture (OFA) of Oracle database? How can we organize the tablespaces in Oracle database to have maximum performance? SYSTEM - Data dictionary tables. DATA - Standard operational tables. DATA2- Static tables used for standard operations INDEXES - Indexes for Standard operational tables. INDEXES1 - Indexes of static tables used for standard operations. TOOLS - Tools table. TOOLS1 - Indexes for tools table. RBS - Standard Operations Rollback Segments, RBS1,RBS2 - Additional/Special Rollback segments. TEMP - Temporary purpose tablespace TEMP_ - Temporary tablespace for s. S - tablespace. 70. How will you force database to use particular rollback segment?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name. 71. What is meant by free extent? A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free. 72.Which parameter in Storage clause will reduce number of rows per block? PCTFREE parameter
Row size also reduces no of rows per block. 73. What is the significance of having storage clause?
We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updating, etc., 74. How does Space allocation table place within a block? Each block contains entries as follows Fixed block header Variable block header Row Header, row date (multiple rows may exists)
PCTEREE (% of free space for row updating in future) 75. What is the role of PCTFREE parameter is storage clause?
This is used to reserve certain amount of space in a block for expansion of rows. 76. What is the OPTIMAL parameter?
It is used to set the optimal length of a rollback segment. 77. What is the functionality of SYSTEM table space?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage. 78. How will you create multiple rollback segments in a database?
- Create a database, which implicitly creates a SYSTEM rollback segment in a SYSTEM tablespace. - Create a second rollback segment name R0 in the SYSTEM tablespace. - Make new rollback segment available (after shutdown, modify init.ora file and start database) - Create other tablespaces (RBS) for rollback segments. - Deactivate rollback segment R0 and activate the newly created rollback segments. 79. How the space utilization takes place within rollback segments?
It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (number of extents is based on the optimal size) 80. Why query fails sometimes? Rollback segment dynamically extent to handle larger transactions entry loads. A single transaction may wipeout all available free space in the rollback segment tablespace. This prevents other using rollback segments. 81. How will you monitor the space allocation?
By querying DBA_SEGMENT table/view
82. How will you monitor rollback segment status? Querying the DBA_ROLLBACK_SEGS view IN USE - Rollback Segment is on-line. AVAILABLE - Rollback Segment available but not on-line. OFF-LINE - Rollback Segment off-line INVALID - Rollback Segment Dropped. NEEDS RECOVERY - Contains data but need recovery or corrupted. PARTLY AVAILABLE - Contains data from an unresolved transaction involving a distributed database. 83. List the sequence of events when a large transaction that exceeds beyond its optimal value when an entry wraps and causes the rollback segment to expand into another extend. Transaction Begins. An entry is made in the RES header for new transactions entry Transaction acquires blocks in an extent of RBS The entry attempts to wrap into second extent. None is available, so that the RBS must extent. The RBS checks to see if it is part of its OPTIMAL size. RBS chooses its oldest inactive segment. Oldest inactive segment is eliminated. RBS extents The data dictionary tables for space management are updated. Transaction Completes. 84. How can we plan storage for very large tables? Limit the number of extents in the table
Separate table from its indexes. Allocate sufficient temporary storage. 85. How will you estimate the space required by a non-clustered tables? Calculate the total header size Calculate the available data space per data block Calculate the combined column lengths of the average row Calculate the total average row size. Calculate the average number rows that can fit in a block Calculate the number of blocks and bytes required for the table. After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table. 86. It is possible to use raw devices as data files and what are the advantages over file system files? Yes. The advantages over file system files are that I/O will be improved because Oracle is bye-ing the kernel which writing into disk. Disk corruption will be very less.
87. What is a Control file? Database's overall physical architecture is maintained in a file called control file. It will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable. 88. How to implement the multiple control files for an existing database? Shutdown the database Copy one of the existing controlfile to new location Edit Config ora file by adding new control filename Restart the database. 89. What is redo log file mirroring? How can be achieved? Process of having a copy of redo log files is called mirroring. This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance. 90. What is advantage of having disk shadowing / mirroring? Shadow set of disks save as a backup in the event of disk failure. In most operating systems if any disk failure occurs it automatically switchover to place of failed disk. Improved performance because most OS volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks. 91. What is use of rollback segments in Oracle database?
They allow the database to maintain read consistency between multiple transactions. 92. What is a rollback segment entry? It is the set of before image data blocks that contain rows that are modified by a transaction.
Each rollback segment entry must be completed within one rollback segment. A single rollback segment can have multiple rollback segment entries. 93. What is hit ratio? It is a measure of well the data cache buffer is handling requests for data. Hit Ratio = (Logical Reads - Physical Reads - Hits Misses)/ Logical Reads. 94. When will be a segment released?
When Segment is dropped. When Shrink (RBS only) When truncated (TRUNCATE used with drop storage option) 95. What are disadvantages of having raw devices? We should depend on export/import utility for backup/recovery (fully reliable)
The tar command cannot be used for physical file backup, instead we can use dd command, which is less flexible and has limited recoveries. 96. List the factors that can affect the accuracy of the estimations? - The space used transaction entries and deleted records, does not become free immediately after completion due to delayed cleanout. - Trailing nulls and length bytes are not stored. - Inserts of, updates to and deletes of rows as well as columns larger than a single data block, can cause fragmentation a chained row pieces.
SQL*Plus Statements 100. What are the types of SQL statement? Data Definition Language: CREATE, ALTER, DROP, TRUNCATE, REVOKE, NO AUDIT & COMMIT. Data Manipulation Language: INSERT, UPDATE, DELETE, LOCK TABLE, EXPLAIN PLAN & SELECT. Transactional Control: COMMIT & ROLLBACK Session Control: ALTERSESSION & SET ROLE System Control: ALTER SYSTEM. 101. What is a transaction?
Transaction is logical unit between two commits and commit and rollback. 102. What is difference between TRUNCATE & DELETE? TRUNCATE commits after deleting entire table i.e., cannot be rolled back. Database triggers do not fire on TRUNCATE
DELETE allows the filtered deletion. Deleted records can be rolled back or committed. Database triggers fire on DELETE. 103. What is a ? Explain the different types of s? is a query, which retrieves related columns or rows from multiple tables. Self - ing the table with itself. Equi - ing two tables by equating two common columns. Non-Equi - ing two tables by equating two common columns. Outer - ing two tables in such a way that query can also retrieve rows that do not have corresponding value in the other table. 104. What is the sub-query?
Sub-query is a query whose return values are used in filtering conditions of the main query. 105. What is correlated sub-query? Correlated sub-query is a sub-query, which has reference to the main query. 106. Explain CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg. select empno, ename from emp where. 107. Difference between SUBSTR and INSTR? INSTR (String1, String2 (n, (m)), INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1. SUBSTR (String1 n, m) SUBSTR returns a character string of size m in string1, starting from n-th position of string1. 108. Explain UNION, MINUS, UNION ALL and INTERSECT? INTERSECT MINUS UNION UNION ALL
- returns all distinct rows selected by both queries. - returns all distinct rows selected by the first query but not by the second. - returns all distinct rows selected by either query - returns all rows selected by either query, including all duplicates.
109. What is ROWID?
ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID. 110. What is the fastest way of accessing a row in a table? Using ROWID. CONSTRAINTS 111. What is an integrity constraint?
Integrity constraint is a rule that restricts values to a column in a table. 112. What is referential integrity constraint?
Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table. 113. What is the usage of SAVEPOINTS?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed. 114. What is ON DELETE CASCADE?
When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed. 115. What are the data types allowed in a table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW. 116. What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2. 117. How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?
Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause. 118. What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint? - To modify the datatype of a column the column must be empty.
- To add a column with NOT NULL constrain, the table must be empty. 119. Where the integrity constraints are stored in data dictionary?
The integrity constraints are stored in _CONSTRAINTS. 120. How will you activate/deactivate integrity constraints?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT. 121. If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE?
It won't, Because SYSDATE format contains time attached with it. 122. What is a database link?
Database link is a named path through which a remote database can be accessed. 123. How to access the current value and next value from a sequence? Is it possible to access the current value in a session before accessing next value?
Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, current value can be accessed. 124. What is CYCLE/NO CYCLE in a Sequence?
CYCLE specifies that the sequence continue to generate values after reaching either maximum or minimum value. After pan-ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum. NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value. 125. What are the advantages of VIEW? - To protect some of the columns of a table from other s. - To hide complexity of a query. - To hide complexity of calculations.
126. Can a view be updated/inserted/deleted? If Yes - under what conditions?
A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible. 127. If a view on a single base table is manipulated will the changes be reflected on the base table?
If changes are made to the tables and these tables are the base tables of a view, then the changes will be reference on the view. Q) What are the Various Master and Detail Relation ships. A) The various Master and a) NonIsolated :: b) Isolated :: c) Cascading ::
Detail Relationship are The Master cannot be deleted when a child is exisiting The Master can be deleted when the child is exisiting The child gets deleted when the Master is deleted.
Q) What is the Difference between a post query and a pre query A) A post query will fire for every row that is fetched but the pre query will fire only once. Q) How do you implement one-to-one, one-to-many and many-to-many relationships while deg tables? A) One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. Q) Can the following Query be Optimized? SELECT CID FROM Course WHERE title LIKE ’S%’ AND min_enroll > (SELECT COUNT(*) FROM Enroll WHERE Enroll.CID = Course.CID); A) The query is very expensive,s ince it is evaluated once for every S course, yes it can be optimised via the followig • Process the outer query without the subquery - CREATE VIEW Supp_Course AS SElect * from Course Where title like 'S%' • Collect bindings – CREATE VIEW Magic AS SELECT DISTINCT CID FROM Supp_Course; • Evaluate the subquery with bindings – CREATE VIEW DS AS sELECT Enroll.CID, COUNT(*) AS cnt FROM Magic, Enroll WHERE Magic.CID = Enroll.CID GROUP BY Enroll.CID; UNION SELECT Magic.CID, 0 AS cnt -- the COUNT patch FROM Magic WHERE Magic.CID NOT IN(SELECT CID FROM Enroll);
• Finally, refine the outer query – SELECT Supp_Course.CID FROM Supp_Course, DS WHERE Supp_Course.CID = DS.CID AND min_enroll > DS.cnt; Q) Which one of the two queries gives the best performance (using sub-q) select person,birth_yr from data where birth_yr in (select birth_yr from data group by birth_yr having count(person) >1 ) (using ) select a.person, b.person, a.birth_yr from data a, data b where a.birth_yr = b.birth_yr and a.person < b.person A) in the above scenario, the subquery gives the best performance since the iterates for each person affecting performance.
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